Apple iOS before 10, when Handoff for Messages is used, does not ensure that a Messages signin has occurred before displaying messages, which might allow attackers to obtain sensitive information via unspecified vectors.
https://support.apple.com/HT207143
http://www.securitytracker.com/id/1036797
http://www.securityfocus.com/bid/92932
http://lists.apple.com/archives/security-announce/2016/Sep/msg00008.html
http://lists.apple.com/archives/security-announce/2016/Sep/msg00002.html